As FIG. 1 illustrates, the path between the Cable Modem Termination System (CMTS) and the Cable Modem (CM) has many attenuation points. The CM needs to transmit enough power such that the signal hits the CMTS at a specific desired power level. The CMTS asks the CM during the ranging process (and station maintenance) to adjust (increase or decrease) its transmitted power so that the received power at the input of the CMTS is equal to the desired value (0 dBmV in FIG. 1).
If the attenuation between the CMTS and CM increases, the CM is asked to transmit more power to keep the received power at the input of the CMTS at the desired level. As more and more attenuation is added on the HFC link (e.g., user puts more splitters inside home), the CM may not be able to transmit enough power to provide a desired received power value at the input of the CMTS (because of its limited power transmission capabilities). These particular modems, which are producing the maximum power output they are capable of and deliver an upstream signal to the input of the CMTS in which the power of the upstream signal at the CMTS is less than the desired received power level may be herein referred to as ‘power-pegged modems.’ This term implies that the transmit power level at the modem is pegged to the modem's maximum output power level. Since the signal power arriving at the CMTS from these power-pegged modems is undesirably low, the ability to receive and demodulate those signals can oftentimes be impaired. Oftentimes, the characteristics of the channel can be modified via modulation profile changes (ex: lower symbol rates, lower-order QAM, etc.) to make it easier to receive and demodulate the arriving low-power signals from the power-pegged modems, but these modified characteristics typically result in a lower bit-rate for all of the modems on the channel. In other words, the non-power-pegged modems are being penalized in their bit-rates because of the existence of power-pegged modems sharing their upstream channel. On typical upstream channels, most of the modems on the channel will not be power-pegged, so the penalties will unfortunately be felt by a large percentage of the modems (which are not power-pegged).
Since the Multiple System Operator (MSO) is required to provide an adequate service to all CMs connected to a CMTS, a modulation profile must be assigned to a particular upstream channel such that all CMs can communicate signals reliably using that modulation profile. The unfortunate case happens when a very small number of CMs on a certain upstream channel are power-pegged, which results in a low-bit-rate modulation profile assigned to that upstream channel by the CMTS. All modems on that upstream channel will unfortunately be penalized and forced to use the low-rate modulation profile for transmission even though most of those modems are non-power-pegged modems and would work with higher-rate modulation profiles. MSOs normally use the low-rate modulation profile approach to solve the problems that occur whenever a few modems on an upstream channel are power-pegged.
Assigning low-rate modulation profiles to upstream channels, where most of the modems are not power-pegged, results in inefficient use of the channel capacity. We propose in this document a solution to this problem.